1. Field of the Invention
This invention relates to sorption cooling systems incorporating a compact, lightweight evaporator and adsorbent bed structure and the use of such sorption cooling systems in vehicles.
2. Description of Related Art
Air-conditioning a car, truck or other motorized vehicle can consume a non-trivial amount of engine motive power. The load from the air-conditioner compressor may affect both fuel efficiency and engine emission levels. Tests conducted by the Society of Automotive Engineers have shown that operation of a standard air-conditioner can decrease vehicle fuel economy by 4.5 miles per gallon, increase NOx emissions by 0.6 grams per mile and increase CO production by 12 grams per mile. These effects became more dramatic as ambient temperature is increased, as humidity is increased, and when a solar load is imposed on the car. In addition to fuel economy and emissions, operation of the air-conditioner gives a clearly perceivable decrease in vehicle performance.
The size of a standard air-conditioner is generally fixed by the required cooling rate. When a vehicle air-conditioner is started, the air temperature and relative humidity may be high. Producing the desired temperature of cooled air often requires chilling the air (whether it is fresh air, recirculated cabin air, or a blend) to below the dew point of the air. This means that large amounts of the air-conditioner cooling capacity may be used to condense water in the air (where it collects in the evaporator), instead of for cooling the air. FIG. 1 illustrates this effect, which depicts the percentage of the total cooling loads attributable to water condensation as a function of relative humidity and temperature. As shown, higher inlet air temperatures and/or humidity may cause over 50% of the air conditioner power to be used in condensing water. Even in dry climates, condensation limits cooling. FIG. 2 illustrates the cyclical operation of a vehicle on a day with an outside ambient temperature of 95–104° F. (35–40° C.) and humidity of 15%. The cycling of humidity in the interior of the vehicle illustrates the effect of water condensation on the rate of interior cooling even in dry climates. This effect can be even further exacerbated in a climate that produces high solar loads.
Due to the issue of cooling loads from humid and/or high temperature conditions, vehicle manufacturers generally design air-conditioners so that they are able to cool a vehicle on start-up in hot and humid climates. Often this results in an over-sized air-conditioning unit, which increases capital costs and weight and unnecessarily wastes energy. This is particularly problematic for vehicles powered by alternative means, such as hybrid, electric, and fuel cells. The use of over-sized air-conditioners also increases pollution since extra fuel is required during start-up when the engine efficiency is still low. An additional problem with conventional vehicle air-conditioners is the odor from bacterial growth in the evaporator.
One approach to increasing air-conditioner efficiency and decreasing its size is to dehumidify the air prior to cooling it with the air-conditioner. For example, PCT Publication No. WO 96/25636 to Denniston discloses dehumidification systems for use in the dehumidification and/or humidification of air prior to its entry into the interior of a vehicle. The publication discloses various bed structures that may be used to dehumidify air and that waste heat from the engine may be used to heat air that is used to regenerate the bed.
U.S. Pat. No. 5,509,275 to Bhatti et al. discloses the use of a desiccant, such as a zeolite, in a desiccant wheel to dehumidify an air stream supplied to the interior of a vehicle. Fresh air is dehumidified in one portion of the wheel and fed to the evaporator of the compression-based air-conditioning system while the other portion of the wheel is regenerated using air heated by waste heat from the engine.
It is believed that these air drying technologies are not currently employed in commercial vehicles due to cost, weight considerations, and inefficiencies that do not always result in a reduction in the size of the air-conditioner. 
Another approach to increasing efficiency of a vehicle cooling system is to employ waste heat generated by the vehicle to operate systems that aid the compression-based air-conditioner, or, in some circumstances, completely obviate the need for the compression-based air conditioner. Approximately 60% of a vehicle's fuel value is lost to the exhaust (33%) and engine cooling (27%). Fuel efficiency could be significantly improved and emissions reduced by employing engine waste heat in the operation of a cooling system. One approach for using such waste heat is to generate electricity from engine waste heat using thermoelectrics and use that electricity to power the air-conditioner. However, the cost, weight and lifetime of such electric generation technology is generally unacceptable. Other cooling technologies include thermoacoustic cooling, ejector cooling and Peltier coolers, but such technologies are generally unproven.
Another method for employing waste heat is to utilize systems comprising an absorber and a reservoir of water, where fresh or recycled air is dehumidified in the absorber and the dehumidified air is subsequently passed over the reservoir of water, where the water evaporates into the dry air to cool and moisten it. Waste heat may be utilized to continuously regenerate an absorbent contained in the absorber. For example, U.S. Pat. No. 5,388,423 to Khelifa discloses such a dehumidifying and subsequent rehumidifying process using an absorbent, such as a zeolite, in providing cooled air to the interior of a vehicle. However, such systems generally require an onboard water storage tank that must be periodically replenished to accomplish the evaporative cooling.
Another approach for utilizing waste heat from the vehicle is the use of a sorption cooling process. A sorption cooling process generally comprises the sequential evaporation, adsorption, regeneration and condensation of a refrigerant to produce cooling. In operation, a working refrigerant is evaporated in an evaporator thereby cooling the surfaces of the evaporator due to the energy required to vaporize the refrigerant. A fluid (e.g., air) is passed over the cooled evaporator surface to cool such fluid. Elsewhere, the evaporated refrigerant is adsorbed by an adsorbent (e.g., a desiccant). The adsorption generally generates heat, which is usually rejected to ambient air. After the adsorbent capacity has been reached, the adsorbent may be regenerated, such as by heating it, to release the adsorbed refrigerant. The desorbed refrigerant is then passed to a condenser to liquefy the refrigerant. The cooling cycle may then be repeated.
Sorption cooling processes are desirable for many reasons. In contrast to standard compression-based air-conditioning systems, the condensation process only requires a small amount of mechanical work to return the refrigerant to its liquid-phase. Thus, the use of a sorption cooling system would decrease the amount of work required to cool the vehicle. Moreover, sorption cooling can provide cooling even when the engine is not running. Also, sorption cooling processes can utilize environmentally friendly refrigerants, such as water. Thus, the use of sorption cooling is a desirable alternative to traditional vehicle air-conditioning processes.
Various attempts have been made to utilize a sorption cooling process in the cooling of a vehicle using liquid-phase absorbents. For example, U.S. Pat. No. 5,896,747 to Antohi discloses the use of an absorbent-based air-conditioning system using engine coolant as the heat source. However, Antohi discloses the use of a liquid-based absorbent, which is not preferred for many reasons. First, liquid-liquid based sorption systems are difficult to separate and purify. Azeotropes and entrainment, among others, can cause the purity of the absorption material to decrease over time, which decreases the efficiency of the process. Additionally, complex pressure and/or temperature variation systems are generally required to separate such solutions, which adds to the complexity of the system. Finally, it is sometimes difficult to achieve the necessary kinetics with the use of liquid absorbents due to limitations in surface area.
Sorption cooling processes using a solid-phase adsorbent are also known. These processes can be categorized into those utilizing gas-phase materials (e.g., hot exhaust air), those utilizing liquid-phase materials (e.g., hot liquids from the engine/radiator), and those utilizing electric sources to regenerate the adsorbent.
U.S. Pat. No. 4,924,676 to Maier-Laxhuber et al., U.S. Pat. No. 5,298,054 to Malik, U.S. Pat. No. 5,333,471 to Yamada and U.S. Pat. No. 5,404,728 to Maier-Laxhuber all disclose the use of a gas-phase materials to heat/regenerate the adsorbent. There are many drawbacks in using gas-phase materials to regenerate the adsorbent, including: (a) high regeneration temperatures to achieve the heat transfer necessary to accomplish the regeneration, (b) increased exhaust pressure drop in the exhaust manifold, which is detrimental to fuel economy and/or exhaust emissions, (c) increased corrosion, (d) use of large heat exchangers, (e) control difficulties related to the high and wide-ranging gas-phase temperatures, (f) water and acid gas condensation in the heat exchangers, and (g) decreased catalytic converter performance, especially on start-up. Also, because of the high temperatures, restrictions are placed on the materials of construction, including the types of adsorbents that may be used. Also, many traditional adsorbents, such as zeolites, require very low operating pressures to be effectively utilized.
U.S. Pat. No. 5,477,706 to Kirol et al. and U.S. Pat. No. 5,768,908 to Tanaka et al. disclose the use of electric heating elements to regenerate the adsorbent. While such systems may be effective for regeneration, electrical regeneration systems add extra complexity to the adsorption system in that a sufficient amount of wires must be placed throughout the adsorbent bed to accomplish the necessary heat transfer to regenerate the absorbent. Electrical systems also add to the complexity of the wiring of the vehicle.
Liquid-phase regeneration systems are also known and various attempts have been made to utilize these materials in a vehicle sorption cooling system. U.S. Pat. No. 4,574,874 to Duran discloses a chemisorption apparatus for use as a vehicle air conditioner. Duran discloses that a high temperature input is applied to the lower parts of a series of elements in one sector of the housing, where a reactant is desorbed from a solid-phase adsorbent and moves to the upper part of the elements, where heat is removed by a fluid medium to an external heat sink. As the series of elements are rotated to another sector of the housing, heat is removed from the lower parts of the elements by heat exchange to a heat sink. Seeking chemical equilibrium, the reactant moves downward to be re-adsorbed in the adsorbent material, and the evaporation in the upper parts of the elements produces a cooling output, which is utilized to cool an output fluid medium.
U.S. Pat. No. 5,619,866 to Sato et al. discloses a multiple-stage adsorption cooling system for providing cooling to a vehicle. Sato et al. disclose that two or more adsorbers function in adsorbing/heating operations while two or more other adsorbers function in desorption operations. Sato et al. provide that the evaporation of water from an evaporator provides the cooling in conjunction with cooling pipes that cool the absorbing systems while heating pipes heat the desorbing system.
Hybrid compression-based air-conditioners assisted by sorption cooling processes are also known. U.S. Patent Publication No. 2004/0093876 to Inagaki et al. discloses an adsorbent cooling system that may be used in vehicles to cool the interior, assist in air-conditioning, and heat the engine. Inagaki et al. disclose a cooling system having an adsorption chamber that includes an adsorbent, preferably a ferroaluminophosphate, which adsorbs an adsorbate, such as water. Inagaki et al. disclose that a traditional compression-based air-conditioner may be used in conjunction with the adsorption process.
Although there are many fuel efficiency and emission reduction reasons to employ sorption cooling instead of conventional vapor compression cooling, it is believed sorption cooling processes are not currently used in vehicle cooling processes for at least three reasons. First, many adsorbents require high regeneration temperatures. The use of high regeneration temperatures necessitates the use of exhaust heat from the vehicle to regenerate the absorbent, as described above.
Second, most absorbents have a relatively low adsorption capacity (e.g., zeolites absorb ˜0.2 grams of water per gram of zeolite), which necessitates the use of large amounts of adsorbent to achieve the desired cooling rates. When a large mass of adsorbent is employed, it is difficult to extract heat therefrom, which further slows down the cooling cycle time, thereby necessitating an even greater adsorbent mass. High desiccant mass in combination with the requirement for high temperature packaging results in a high thermal mass of the entire system. A high thermal mass may lead to high parasitic energy losses of the absorption system. This effect may be compounded with the use of traditional absorbents, such as zeolites, which normally have water heats of adsorption approaching twice the heat of vaporization of water. The net effect of such a high thermal mass and heats of absorption results in a system that must reject twice as much heat as cooling that is produced. This increases the required blower power for the cooling step and further reduces the COP (coefficient of performance).
Third, since a high volume of adsorbent is generally required, an increased number of adsorbent beds is generally used, which translates to an increased amount of high temperature materials of construction, larger area evaporators and condensers, and larger blowers that must be used to operate the system. Such large adsorbent cooling systems are impractical and too expensive to be utilized as a vehicle air-conditioning system. Thus, despite the promise of sorption cooling process for vehicle air-conditioning, the technology has not achieved commercial success.
Another application of waste heat is in providing a cooled fluid to a building. Today, many standard industrial processes use energy cogeneration to increase the efficiency of their heating and electrical generation processes. Cogeneration is the simultaneous production of electricity and useful heat, usually in the form of either hot water or steam, from one primary fuel, such as a fossil fuel. While cogeneration is useful, much of the heat produced by the electrical and/or heat generation techniques goes unused. Moreover, many industrial applications also desire the production of a cooled fluid to utilize in cooling applications, such as air-conditioning of buildings. Therefore, there has been effort to utilize cogeneration waste heat to also generate a cooled fluid. This energy concept is called trigeneration. While attractive, there are many hurdles to producing an efficient trigeneration cooling system, including the production of an efficient adsorption system capable of producing a cooled fluid from the waste heat of a cogeneration system.